It has long been known to thermally crack hydrocarbon to produce olefins and other lighter hydrocarbon products.
Typically, a thermal cracking furnace is comprised of a firebox and a plurality of coils that extend through the firebox. A hydrocarbon feedstock is introduced into the cracking furnace and elevated to high temperatures, e.g. 1600.degree. F. and quenched to a reaction temperature to provide a yield of cracked products. However, the nature of the thermal cracking process causes coke and tar to form along with the desired products. From the beginning of the practice of thermal cracking, fouling of the coils resulting from coke and tar generation has been a serious problem. When the coils are fouled by coke and tar the furnace must be taken out of service to clean or replace the tubes.
Light hydrocarbons such as ethane are a common and often preferred feedstock. However the high heat of cracking of light hydrocarbon feedstocks poses design constraints and the fouling characteristics of coke from the cracking of the light hydrocarbon feedstocks is particularly troublesome.
Furthermore, as the thermal cracking technology advanced, a trend to high severity cracking occurred to achieve either improved yields or increased selectivity to the desired ultimate product. As a result, thermal cracking furnaces having small diameter, short length coils and a concentration of radiant burners along the furnace walls facing the coils were developed for high severity cracking to attain higher olefin selectivity. Practice has shown that at high severity coking problems become more pronounced.
A further development was the application of floor firing of thermal cracking furnaces. Although many benefits attend floor firing, experience indicated that deleterious localized coking often resulted from floor firing.
The conventional wisdom now prevailing in thermal cracking is that short residence time, high severity cracking will produce the highest selectivity and olefin yield. However, under high severity cracking conditions, particulary in conjunction with total floor firing, the coking problems increase and the operating run length consequently decreases causing shorter effective operational availability and curtailed equipment life.